There is growing scientific awareness that understanding the population biology of animal-pathogen interactions is very relevant for predicting human health risks from emerging infectious diseases. This application aims to develop a model insect-pathogen system to examine how seasonal animal migrations, and associated changes in host behavior, reproduction and immunity, affect pathogen spread. Many animal species undergo seasonal movements to track changes in climate or resources;these movements often have significant energetic costs and are accompanied by major physiological changes. Migratory animals can also harbor infectious diseases with known human health risks - yet little is known about the general consequences of long-distance migrations for host-pathogen interactions. I will approach this issue first empirically through field studies by evaluating how host breeding densities and duration of time spent in a given habitat affect parasite transmission and accumulation in the hosts'environment. To examine how the costs of infection impact host migratory success, I will incorporate field data analysis with stable isotope markers to infer the natal origins of hosts captured along migratory routes and at their wintering sites. Experimental work will test how physiological changes that precede migration influence host immunity and tolerance to infection. Lastly, I will develop mathematical models to explore how host migration distances and movements influence infectious disease dynamics. Models will examine how animal migratory strategies affect host-parasite dynamics, and how infection rates influence the relative costs and benefits of host migration, with applications to a broad range of animal-pathogen systems. Spatio-temporal information on host movement behaviors is necessary to predict the spread of infectious diseases in migratory animals acting as natural reservoirs of human infectious diseases. Many human emerging infectious diseases result from exposure to zoonotic pathogens harbored by migratory animals, such as avian influenza virus, West Nile virus, Lyme disease, and SARS-like coronaviruses: hence, knowledge of pathogen movement is crucial to assess spatiotemporal incidence patterns. Understanding processes by which animal migration affects parasite transmission and host immunity are therefore crucial for predicting how loss of migrations will influence infectious disease dynamics. Using a highly tractable insect-pathogen model system, my work will shed light on mechanisms that may be important for predicting future changes in the complex relationships between host populations and emerging infectious diseases.